The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In dual clutch transmissions (DCT's) and manual transmission automation (MTA), gear ratio changes are typically accomplished by double acting hydraulic cylinders. One side of a cylinder is pressurized with hydraulic fluid while the other side is exhausted to move a piston and associated shift rail, fork and synchronizer clutch in one direction to engage one gear and the opposite activity engages another gear. The double acting cylinder is configured to also provide a center or neutral position.
One of the acknowledged features and benefits of dual clutch transmissions is their ability to rapidly shift gears. Such rapid gear shifts actually are achieved when one of the dual clutches engaging the current gear ratio on one countershaft is released and the other clutch on the other countershaft is engaged, the newly selected gear ratio having been previously pre-engaged by the process described above.
Notwithstanding such gear ratio pre-selection and its attendant shift time reduction, there is still a desire and demand to even more rapidly change gear ratios. The speed of such gear ratio changes may be increased by many commonly known approaches: increasing the hydraulic pressure, increasing the flow rate of the hydraulic fluid and increasing the size of the hydraulic cylinder. Unfortunately, all of these approaches carry with them the essential certainty of increased noise of a very noticeable and objectionable type. The noise will not be, for example, a continuous or low frequency sound which might not be noticeable given the other sounds from the vehicle but will be a distinct and abrupt impact or clunk as the piston, shift rail and gear come to a sudden stop upon reaching the limit of travel and engagement.
This problem has not gone unrecognized and significant effort has been expended to rectify it. One of the more accepted solutions is referred to as apply pressure profiling. This involves controlling or adjusting the hydraulic pressure applied to a piston and reducing it as the piston approaches its travel limit to slow it in order to minimize the noise generated as it stops. Clearly this solution to gear ratio change noise is a compromise as it results in slower average piston travel and thus slower gear engagement. Furthermore, it significantly increases the complexity of the electronic control and driver circuitry as modulating control of the pressure of the hydraulic fluid must now be provided. Finally, if the travel of the hydraulic piston is very short, there is simply not enough time to achieve effective pressure profiling due to the inertia of the mechanical components of the system.
From the foregoing, it is apparent that noise reduction improvements in the art of gear ratio change mechanisms for dual clutch and other transmissions would be desirable and the present invention is so directed.